1. Field of the Invention
The present invention relates to the thermal insulation of a furnace such as an optical fiber drawing furnace, a sintering furnace for a glass preform for an optical fiber glass preform; a semiconductor pulling-up furnace and the like, particularly those used at a high temperature, for example, higher than 1000.degree. C. Hereinafter, this type of furnace will be generally referred to as a high temperature furnace.
2. Description of the Related Art
When the high temperature furnace is thermally insulated in order to reduce heat loss, usually a carbon felt, a ceramic fiber felt or the like is used as an insulation of the furnace. In order to prevent the fibers in the felt from scattering, such the insulation is contained in a cylindrical container, or shaped into a cylindrical form of molded felt.
When a shaped felt is used as the insulation in the furnace, as shown in FIG. 1, the shaped felt 3 is disposed around a heater 2 which is installed around a muffle tube 1. With the use of the shaped felt as described above, the heat loss through a container containing the felt can be advantageously eliminated since the container can be omitted. It may also be possible to coat the shaped felt with carbon cement in order to prevent the fibers from scattering.
Further, the shaped felt as shown in FIG. 2 has been proposed. This has a three layer configuration consisting of the three shaped felt parts 3a, 3b and 3c. In this configuration, the innermost shaped felt part 3a, which is heated to the highest temperature, is made of a better insulating material with a higher density, while the outermost layer 3c, which is not heated to as high of a temperature, is made of a material with a smaller heat capacity and a smaller density. With such a configuration, a combination of the better thermal insulation with the smaller heat capacity can be achieved.
In order to reduce the heat loss, other methods have also been proposed. In one method, a cylindrical container containing inorganic powder such as carbon powder or zirconia powder is used. In another method, a sheet material made of carbon, molybdenum or the like which reflects the infrared rays is used in order to decrease the heat loss due to thermal radiation.
In principle, optical fibers to semiconductors can be most efficiently produced when a large amount of raw material is treated in the furnace. However, it is not practical to treat a large amount of material in the absence of effective thermal insulation, since the furnace requires a larger scale facility and also consumes a larger amount of electrical energy.